Large-area organic solar cells by accelerated blade coating

Large-area photovoltaic devices have been fabricated using the blade coating technique. In this study, the use of accelerated blade motion in this technique significantly improved the thickness uniformity of blade-coated layers of polymer solar cells on an A4 glass substrate. Two types of active layers, P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, were studied. For the P3HT:PC₆₁BM film, a thickness of 221 ± 14 nm was realised in a 12 × 15 cm² active region with a coating blade acceleration of 8 mm/s². For the POD2T-DTBT:PC₇₁BM film, a thickness of 98 ± 6 nm was realised with a coating blade acceleration of 10 mm/s². Ten cells, each measuring 0.9 cm × 12 cm and monolithically fabricated, were connected in series, yielding a total active area of 108 cm². The power conversion efficiency of the resulting 10-cell module was 2.66% and 3.64% for P3HT:PC₆₁BM and POD2T-DTBT:PC₇₁BM, respectively. The blade coating technique involving the accelerated blade motion is therefore useful for fabricating low-cost large-area organic solar cells, and it may be a promising alternative for the commercialisation of organic solar cells.     

Vacuum deposited ternary mixture organic solar cells

Ternary mixtures of photo-active organic materials are an intuitive approach to achieve enhanced photocurrent in organic solar cells (OSCs). In this work, we study ternary mixtures of vacuum deposited small molecules, complementing the recent surge of interest in solution processed ternary OSCs. The mixed layer composition is systematically varied to study all possible film configurations, and the resulting OSCs are successful in harvesting photocurrent from all three components to grant broad spectral photoresponse. However, the performance of the ternary OSC is generally less than the binary OSC, largely due to reduced fill factors. By examining ternary OSC transient photocurrents and multi-donor planar heterojunction devices, we demonstrate that the ternary OSC is strongly affected by the energy levels of its constituent materials, with small differences in the two donor materials’ highest occupied molecular orbitals degrading hole transport. The results stress the importance of fine molecular engineering for ternary OSCs, and further hint that the enhancements commonly observed in solution processed ternary OSCs may in part be associated with morphological variations that are not present in vacuum deposited OSCs. The research verifies that, by designing small molecules with specific energy levels, ternary OSCs provide an alternative pathway to low cost, high efficiency photovoltaics in lieu of more complicated device architectures.     

Time-independent charge carrier mobility in a model polymer:fullerene organic solar cell

The technique of photo-CELIV (charge extraction by linearly increasing voltage) is one of the more straightforward and popular approaches to measure the faster carrier mobility in measurement geometries that are relevant for operational solar cells and other optoelectronic devices. It has been used to demonstrate a time-dependent photocarrier mobility in pristine polymers, attributed to energetic relaxation within the density of states. Conversely, in solar cell blends, the presence or absence of such energetic relaxation on transport timescales remains under debate. We developed a complete numerical model and performed photo-CELIV experiments on the model high efficiency organic solar cell blend poly[3,6-dithiophene-2-yl-2,5-di(2-octyldodecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-naphthalene] (PDPP-TNT):[6,6]-phenyl-C₇₁-butyric-acid-methyl-ester (PC70BM). In the studied solar cells a constant, time-independent mobility on the scale relevant to charge extraction was observed, where thermalisation of photocarriers occurs on time scales much shorter than the transit time. Therefore, photocarrier relaxation effects are insignificant for charge transport in these efficient photovoltaic devices.     

Highly efficient inverted rapid-drying blade-coated organic solar cells

Efficient inverted bulk-heterojunction (BHJ) poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PCBM) organic solar cells fabricated by rapid-drying blade-coated were demonstrated. Optimized self-organization interpenetration networks and donor/acceptor domain sizes were obtained while maintaining the smooth surface morphology. By integrating with low-temperature-processed sol-gel ZnO electron extraction layer, power conversion efficiency (PCE) up to 4.4% under AM1.5G 1 sun illumination is achieved, compared to fast drying but low efficiency (1.2%) and high efficiency but with long-time solvent annealing treatment (4.3%) control cells deposited by spin coating in chlorobenzene (CB) and 1,2-dichlorobenzene (DCB) solution, respectively. The novel deposition technique reveals a promising process for highly efficient, high throughput, stable morphology organic solar cells fabrication.     

Efficient ternary organic solar cells based on immiscible blends

Organic photovoltaic cells based on ternary blends of materials with complementary properties represent an approach to improve the photon-absorption and/or charge transport within the devices. However, the more complex nature of the ternary system, i.e. in diversity of materials' properties and morphological features, complicates the understanding of the processes behind such optimizations. Here, organic photovoltaic cells with wider absorption spectrum composed of two electron-donor polymers, F8T2, poly(9,9-dioctylfluorene-alt-bithiophene), and PTB7, poly([4,8-bis[(2′-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2′-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]), mixed with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM) are investigated. We demonstrate an improvement of 25% in power conversion efficiency in comparison with the most efficient binary blend control devices. The active layers of these ternary cells exhibit gross phase separation, as determined by Atomic Force Microscopy (AFM) and Synchrotron-based Scanning Transmission X-ray Microscopy (STXM).     

A two-dimension-conjugated small molecule for efficient ternary organic solar cells

Ternary organic solar cells (OSCs) are burgeoning as one of the effective strategies to achieve high power conversion efficiencies (PCEs) by incorporating a third component with a complementary absorption into the binary blends. In this study, we presented a new two-dimension-conjugated small molecule denoted by DR3TBDTTVT, which alone gave rise to a best PCE of 5.71% with acceptor PC₇₁BM as active layer. Given the complementary absorption with PTB7-Th, DR3TBDTTVT was doped into (PTB7-Th:PC₇₁BM)-based binary blends, and ternary OSCs were developed. The ternary OSCs with 10 wt% of DR3TBDTTVT displayed improved hole-mobility, reduced device resistance and better phase separation of active layer, thus leading to an impressive PCE of 7.77% with open-circuit voltage of 0.77 V, short-circuit density of 14.52 mA cm⁻² and fill factor of 70.3%. Ternary OSCs well make up for the light-harvesting insufficiency of binary OSCs, and this research provides a new material for the improvement of PCEs for single-junction OSCs.     

High-performance alloy-like ternary organic solar cells with two compatible non-fullerene acceptors

Ternary blending is one of the effective strategies to modulate the blend film morphology for achieving high efficiency organic solar cells (OSCs). In this work, high-performance ternary OSCs are fabricated by introducing a non-fullerene acceptor, namely IDTP-4F into the PM6:Y6 binary system to enhance the device performance. Detailed investigations indicate that IDTP-4F can form an alloy phase with Y6, resulting in the optimized morphology, which can facilitate the charge transport and reduce recombination, leading to enhanced open-circuit voltage (V_oc) and fill factor (FF) simultaneously. Consequently, the optimized ternary OSCs exhibit an excellent power conversion efficiency (PCE) of 17.1%, which is much higher than that of PM6:Y6 binary OSCs (15.9%). These results indicate that combining two compatible non-fullerene acceptors is an effective strategy to fabricate high efficiency ternary OSCs.     

Improved mobility and lifetime of carrier for highly efficient ternary polymer solar cells based on TIPS-pentacene in PTB7: PC71BM

Highly efficient ternary polymer solar cells (T-PSCs) realized by the improved mobility and lifetime of carrier in PTB7: PC₇₁BM: TIPS-pentacene blends were fabricated. By adjusting the weight ratios of third component TIPS-pentacene in the binary PTB7: PC₇₁BM blends, we found that the short circuit current and fill factor (FF) were simultaneously enhanced, resulting in a maximum power conversion efficiency (PCE) of 8.09% with 21.3% improvement. The improved photovoltaic performance of T-PSC was mainly due to the enhanced light absorption, energy level cascading, optimized blend morphology, and increased hole mobility. It was also found that the incorporation of TIPS-pentacene increased the average hole lifetime, ensuring efficient hole transport and collection with suppressed bimolecular recombination, contributing to the photocurrent. Additionally, the low thickness dependent row-off of FF indicates TIPS-pentacene is a promising third component for the realization of thick film T-PSC. The improved PCEs were obtained as well for other ternary donor: acceptor: TIPS-pentacene systems, demonstrating that the incorporation of TIPS-pentacene is a wide practicable methodology for the development of highly efficient T-PSCs.     

Ternary organic solar cells based on non-fullerene acceptors: A review

Organic solar cells (OSCs) have reached their second golden age in recent two years with a boosted number of publications. Non-fullerene acceptor (NFA) materials have become a rising star in the field which are widely applied in organic solar cells because of their excellent optoelectronic properties, such as strong light-harvesting ability and tunable energy level. Unlike the low synthetic flexibility and high production cost of fullerene materials, NFAs exhibit flexible structures, and relatively low fabrication costs. Recently, the ternary strategy has become another hot research topic in the field, which introduces a third component into the binary host system for OSCs. The application of a ternary strategy can break the limits of light absorption brought by the host system, improve the morphology and energy level alignment for the active layer and thus improved the efficiency of organic solar cell devices. Benefiting from the advancement in both NFA and ternary strategy, the power conversion efficiency (PCE) of organic solar cell has exceeded over 17.5% to date. A comprehensive review of the recent progress in NFA based ternary OSCs (TOSCs) is needed in the field. Herein, this review mainly focuses on recent research on ternary organic solar cells using NFA materials during the last two years. Firstly, device physics and frequently used active materials in NFA based TOSCs are summarized and discussed. Then, the recent reported high-performance NFA based TOSCs are reviewed. Finally, the outlook and future research direction in the field are proposed. This review aims to provide an insight into NFA based TOSCs and help researchers to explore the full potential of OSCs.     

A simple and effective method via PH1000 modified Ag-Nanowires electrode enable efficient flexible nonfullerene organic solar cells

Flexible transparent electrodes (FTEs) play an important role in determining the performance of flexible organic solar cells (OSCs), Ag-nanowires (AgNWs) with the unique merits of high conductivity, excellent flexibility, and good thermal stability has been taken into more consideration in fabricating highly efficient FTEs. However, the pristine AgNWs film usually suffers a huge surface roughness and incompatibility with the organic absorption layer, thus always leads to a poor power conversion efficiency (PCE). Herein, we demonstrated a simple and effective way through employing poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PH1000) to modify the surface of AgNWs to prepare high-quality FTEs. Based on the PH1000 (100 nm)/AgNWs FTEs, the optimized flexible OSC with PM6:Y6 as active layer exhibits a highest PCE of 12.71%, with an open-circuit voltage (V_oc) of 0.814 V, a short-circuit current (J_sc) of 22.61 mA/cm², and a fill factor (FF) of 0.691, respectively. Which is much higher than the PCE (7.20%) of pristine AgNWs FTEs based device. The enhanced device performance was attributed to the improved morphologies both of the FTEs and the active layers, more effective charge transport and collection efficiency, as well as the decreased charge recombination properties. This work provides an efficient way to fabricate high-quality FTEs and realize efficient flexible OSCs.     

High performance airbrush spray coated organic solar cells via tuning the surface tension and saturated vapor pressure of different ternary solvent systems

In this work, thieno [3,4-b] thiophene/benzodithiophene (PTB7): [6,6]-phenyl C71-butyric acid methyl ester (PC₇₁BM) based organic solar cell (OSC) with a new record of power conversion efficiency (PCE) of ∼7.62% has been realized using airbrush spray (AS) coating method in air ambient which can be well compatible with large-scale fabrication. By investigating the physical mechanism of AS coated blend films, a series of ternary solvent systems (TSS) are used to simultaneous optimize the surface tension and the saturated vapor pressure of solution. Therefore, different TSS further controls the morphology of PTB7:PC₇₁BM blend films precisely and systematically. It is elucidated that the chlorobenzene (CB)/o-Xylene (o-Xy)/1, 8-diiodoctane (DIO) TSS with a ratio of 37:60:3 vol.% could lead to a homogeneous surface morphology with a decreased aggregation domain size of active layer. In addition, the high fill factor, increased PC₇₁BM absorption and internal quantum efficiency indicate the formation of bicontinuous interpenetrating and fully percolated networks with nanostructured phase separation in BHJ blend films. Ultimately, the AS coated OSCs based on the TSS of CB/o-Xy/DIO gains a 34% enhancement in PCE, compared with the conventional CB/DIO solvent based OSCs.     

Utilizing Forster resonance energy transfer to extend spectral response of PCDTBT:PCBM solar cells

Light harvesting in the near-infrared part of the solar spectrum is important to achieve high efficiency polymer solar cells (PSCs). In this work, we demonstrate that we take an existing polymer:fullerene blend and extend its spectral response into the near-IR region by adding a small amount near-IR absorbing dye in the blend. The polymer studied in this work is Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT). By adding only 2.5% squaraine dye to the blend, we were able to extend the spectral response of the resulting devices 100 nm into the near-IR spectral region. We show that the enhanced light harvesting is due to efficient Forster resonance energy transfer (FRET) between PCDTBT and the squaraine dye, resulting in an increase in power conversion efficiency. This type of ternary polymer solar cells is unique in that it allows the use of a small amount of selected dyes to extend light harvesting in infrared region.     

High conductive PEDOT via post-treatment by halobenzoic for high-efficiency ITO-free and transporting layer-free organic solar cells

The 4-halobenzoics (XBA) including 4-fluorobenzoic acid (FBA), 4-chlorobenzoic acid (CBA), 4-bromobenzoic acid (BBA) and 4-iodobenzoic acid (IBA) have been applied to modify poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) by simply spin-coating solvent treatment. A universal and significant improvement in the conductivity of PEDOT:PSS (from ∼1 S/cm to more than 3300 S/cm) has been achieved by XBA modification, which results from the acid XBA-induced phase segregation, depletion of PSS chains and the conformational change of the conductive PEDOT chains. Especially, the oxidation CBA treated PEDOT:PSS exhibits low sheet resistance of 43 Ω/sq and transparency of over 80% in the visible range, which are comparable to those of indium tin oxide (ITO). The modified PEDOT:PSS has been facilely applied as the transparent anode for high-efficiency ITO-free organic solar cell device without hole transport layer. The high power conversion efficiency of 7.9% has been achieved by employing CBA treated PEDOT:PSS as anode for ITO-free device based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):[6,6]-phenyl-C71 butyric acid methyl ester (PC₇₁BM), which is comparable to the corresponding devices based on the traditional ITO anode.     

Efficient PTB7-Th:Y6:PC71BM ternary organic solar cell with superior stability processed by chloroform

Blend morphology is crucial for the efficiency and stability of organic solar cells. Exploring and understanding the correlations between is meaningful and greatly desired. In this work, based on polymer donor (PTB7-Th), fullerene and non-fullerene acceptors (PC₇₁BM and Y6), we systematically study the influence of ternary strategy and solvent system on device performance and stability. It is found that insufficient and excessive phase separation of blend could result in the depressed performance of corresponding devices. Appropriate phase separation/blend morphology can be achieved by utilizing a ternary strategy or suitable solvent. Chloroform-processed ternary blend PTB7-Th:Y6:PC₇₁BM delivers efficiency of 9.55%, with dramatically enhanced J_SC of 24.68 mA cm⁻² due to optimized absorption, blend morphology and optoelectronic properties. More importantly, superior device stability is demonstrated for the optimal ternary device under both thermal stress and maximum power point operation, by maintaining 80% of initial efficiency at 85 °C for 880 h and presenting almost zero efficiency decay in 200 h under MPP operation.     

High performance organic solar cells enabled by an iodinated additive

Additive engineering is a simple and effective strategy to enhance the efficiency of organic solar cells (OSCs). However, traditional additives such as 1,8-diiodooctane (DIO) or 1-chloronaphthalene (CN), suffer from inferior stability, concentration sensitivity, and need additional thermal treatments, which are not desirable for industrial application. Here we introduce a simple, effective and versatile solid additive 1,3-diiodobenzene (1,3-DIB) into the OSCs. In comparison to the control devices, the 1,3-DIB treated OSCs exhibit significantly improved performance with a power conversion efficiency (PCE) of 16.90% for polymer OSCs and a PCE of 14.35% for binary all-small-molecule OSCs. Mechanism studies reveal that 1,3-DIB can improve charge transport and extraction, decrease charge recombination, enhance crystallinity and improve the phase separation. Furthermore, no thermal annealing is needed in PM6:Y6 based OSCs and the 1,3-DIB treated devices show excellent stability and reproducibility in both polymer and small molecule OSCs. Our results demonstrated that additive engineering is a powerful method to enhance the OSC performance.     

Ternary organic solar cells with PCEs of up to 16.6% by two complementary acceptors working in alloy-like model

Nowadays, improving the power conversion efficiency (PCE) of organic solar cells (OSCs) is still the most concerned issue. Ternary strategy can effectively improve the PCE while maintaining the single junction device structure from well-developed material systems. Herein, a brominated non-fullerene acceptor of ITC-2Br1 is used as the third component to optimize the efficient PM6:Y6 binary system with highly complementary absorption and higher LUMO value as compared with Y6. With 15% wt ITC-2Br1 in the ternary system, the device achieved a best PCE of 16.6%, which is significantly higher than that of its binary counterpart (15.6%). Through the study of neat and blend films, it is found that ITC-2Br1 and Y6 show good miscibility and the absorption of ternary blend film almost cover the whole range from visible and near-IR regions. The optimized ternary device shows an alloy-like working model with improved carrier lifetime and enhanced charge collection efficiency. This work provides a good example for the third component selection in ternary system, and the ternary strategy is a facile and effective method to boost the device performance of OSCs.     

Improving charge transport and suppressing charge recombination in small molecule ternary solar cells via incorporating Bis-PC71BM as a cascade material

The development of small molecule organic solar cells (SMOSCs) has attracted considerable attention and achieved comparable power conversion efficiency (PCE) with polymer solar cells. Here, we demonstrate a bulk heterojunction (BHJ) small molecular solar cell with PCE of 5.31% by incorporating Bisadduct of phenyl-C₇₁-butyric acid methyl ester (Bis-PC₇₁BM) as an additional acceptor material into the host binary blend of 2-[4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-butyl-N-phenylamino)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-1-en-1-olate (SQ-BP): [6,6]-phenyl C₇₁ butyric acid methyl ester (PC₇₁BM). The short circuit current (J_SC) and the fill factor (FF) of ternary SMOSCs are improved by decreasing the carrier recombination loss, increasing exciton dissociation and enhancing the carrier transport. The transient photovoltage (TPV) measurement indicates that the gradient HOMO energy alignment suppresses the charge recombination and leads to the increased charge carrier lifetime in ternary SMOSCs. As a result, the PCE of ternary devices with 5 wt% Bis-PC₇₁BM is about 20% greater than that of SQ-BP: PC₇₁BM based binary SMOSCs.     

Carbon nanotubes for organic/inorganic hybrid solar cells

This article describes the material properties and thin film forming strategies for carbon nanotubes. It summarizes the developments and the challenges related to doping and reviews the highlights over the past decade about organic/inorganic hybrid solar cells using carbon nanotubes (CNTs) CNTs. Replacing the indium tin oxide electrode by CNT spiderwebs have displayed solar cell efficiencies of about 3–4% for organic bulk heterojunction devices, enabling a cost effective fabrication of organic solar cells by roll-to-roll process. Investigations on SWNT/Si hybrid solar cells with efficiency of 17% demonstrate the possibility of wide range applications of SWNTs in organic/inorganic hybrid solar cells.     

All-solution processed organic solar cells with top illumination

All-solution processed organic solar cells with inverted device architecture were demonstrated. Devices contain opaque bottom electrodes and semitransparent top electrodes, resulting in top illuminated devices. Nanoparticles-based Ag ink was used for inkjet printing both top and bottom electrodes. Semi-transparent top electrode consists of high conductivity PEDOT:PSS and Ag current collecting grids. Printed electrodes were compared to evaporated Ag electrodes (both top and bottom) and to ITO electrode in terms of transmittance, roughness, sheet resistance and device performance. All-solution processed devices with top illumination have average PCE of 2.4%, using P3HT:PCBM as photoactive layer. Top-illuminated devices with inverted architecture and bottom-illuminated device with conventional architecture, containing the identical layers, but in the reverse sequence, were then compared. Performed studies have revealed an advantage of inverted cell architecture.     

Thermal analysis of organic solar cells using an enhanced opto-thermal model

In this paper, an opto-thermal model is presented in order to specify the dominant thermal phenomena in organic solar cells (OSCs), as rather low efficiency photovoltaic devices. This model is capable of predicting the amount of optical heat generation (Q_{th_opt}), also the transient and steady state thermal behavior of an organic photovoltaic cell combining both the optical and thermal models. In a typical organic solar cell, Q_{th_opt} plays a significant role in heating up the device while the electric heat generation (Q_{th_elec}) does not effectively have such a role. Developing an optical model for a solar cell, Q_{th_opt} can be determined in every position of the device; also, the contribution of each layer in heat generation is precisely specified. The device thermal behavior is predicted by feeding the thermal model with Q_{th_opt}. This is done for an organic solar cell with a typical architecture and it is shown that thermal convection and radiation are two determinative thermal phenomena while conduction plays a minor role; furthermore, the electrodes, Aluminum (Al) cathode and Indium Tin Oxide (ITO) anode, are two strong light absorbers which contribute to more than 80% of optical heat generation. Assuming Stefan–Boltzman radiation loss, the temperature rise for a typical single junction OSC is estimated under different conditions. The device temperature rise might be even larger for other architectures consisting of several layers depending on their thicknesses and absorption coefficients. This temperature increase enhances the OSCs’ efficiency while degrading the lifetime. The model can be applied to thermal analysis of other types of photovoltaic cells and optoelectronic devices with minor modification.